In the process of doping semiconductor wafers, a common technique is to move the wafers relative to a fixed ion beam along a controlled path so that the ion beam density impacting the wafers is uniform and can be controlled by modifying the trajectory of the wafers in its movement relative the ion beam.
The components of an ion implanter include an ion source, acceleration apparatus for accelerating ions emitted by the source to a desired energy level, and analysis means such as a mass analyzer for diverting from the ion beam all but a well specified mass of ions used for ion beam treatment of a workpiece such as the semiconductor wafer.
In order to effectively transport the ion beam through drift spaces and conventional optics elements, the intensity distribution within the ion beam should be close to uniform.
One technique that has been commonly used to help control the shape of the ion beam in its travel path to the silicon wafer is a space charge neutralization technique wherein negatively charged particles are injected into the ion beam at a controlled density to neutralize the space charge of the beam. This process tends to reduce space charge induced repulsion which causes the ion beam to become more diffuse as ions within the beam move along their travel paths to the impact location.
In the prior art, typical techniques for maintaining control over the ion beam cross section have also included beam focusing devices which deflect ions within the ion beam in efforts to maintain control over the ion beam along the beam trajectory. All such efforts have involved use of beam modelling based upon theoretical effects expected due to space charge repulsion of a particular configuration ion beam. More specifically, prior art techniques are known and have been utilized for shaping ion beams having rectangular and circular cross sections since diffusion processes from these shape ion beams have been theoretically modelled.
Theoretical modelling of the charge distribution and uniformity of an ion beam charged configuration assumes that no electric field gradient exists in the long dimension of a rectangular beam. Uniform beam density in a rectangular beam results in an electric field gradient distribution having sharp discontinuities at the edges of the beam. This is an impossible situation to maintain over long ion beam trajectories. This raises the question of what beam distribution and resultant space charge gradient is in fact experienced by a rectangular beam and also raises the question of what countering steps in the form of ion beam focusing can be implemented on a rectangular beam.
The sharp nonuniformities in electrical gradient distribution experienced with a rectangular beam are not encountered with a circular beam. The current density experiences a uniform fall off as the radius to the center of the beam increases. This makes the ion beam modeling quite simple but results in low current and thus less effective ion implantation.